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A solar-powered airplane that developers hope to eventually pilot around the world took off early on Friday from San Francisco Bay on the first leg of an attempt to fly across the United States with no fuel but the sun's energy.

The plane, dubbed the Solar Impulse, departed shortly after 6 a.m. local time from Moffett Field, a joint civil-military airport near the south end of San Francisco, heading first to Phoenix on a slow-speed flight expected to take 15 to 20 hours.

The spindly looking plane barely hummed as it took flight in the still northern California morning as the sun was just beginning to peek out over the Santa Cruz Mountains to the east.

After additional stops in Dallas, St. Louis and Washington, D.C., with pauses at each destination to wait for favorable weather, the flight team hopes to conclude the plane's cross-country voyage in about two months at John F. Kennedy International Airport in New York.

Swiss pilots and co-founders of the project, Bertrand Piccard and Andre Borschberg, will take turns flying the plane, built with a single-seat cockpit, with Piccard at the controls for the first flight to Arizona. He is tentatively scheduled to land in Phoenix at 1 a.m. local time on Saturday.

The project began in 2003 with a 10-year budget of 90 million euros ($112 million) and has involved engineers from Swiss escalator maker Schindler and research aid from Belgian chemicals group Solvay - backers who want to test new materials and technologies while also gaining brand recognition.

Project organizers say the journey is also intended to boost worldwide support for the adoption of clean-energy technologies.

"I hope people understand the potential of this technology and use it on the ground," Borschberg, who flew for the Swiss Air Force for more than 20 years, told reporters as Piccard suited up for the flight nearby. "If we don't try to fly today using renewable energy, we never will."

With the wingspan of a jumbo jet and the weight of a small car, the Solar Impulse is a test model for a more advanced aircraft the team plans to build to circumnavigate the globe in 2015.

The plane made its first intercontinental flight, from Spain to Morocco, last June.

Solar cells built into wings

The aircraft runs on about the same power as a motor scooter, propelled by energy collected from 12,000 solar cells built into the wings that simultaneously recharge batteries with a storage capacity equivalent to a Tesla electric car.

In that way, the Solar Impulse can fly after dark on solar energy generated during daylight hours, and will become the first solar-powered aircraft capable of operating day and night without fuel to attempt a U.S. coast-to-coast flight.

But the plane, which from a distance resembles a giant floating insect in the sky, is unlikely to set any speed or altitude records. It can climb gradually to 28,000 feet (8,500 meters) and flies at an average pace of just 43 miles per hour (69 km per hour).

The current plane was designed for flights of up to 24 hours at a time, but the next model will have to allow for up to five days and five nights of flying by one pilot - a feat not yet accomplished.

Meditation and hypnosis were part of the training for the pilots as they prepared to fly on very little sleep.

Asked about the downside of solar-powered flight at a news conference in March to unveil the current plane, Piccard acknowledged there was a price paid for the tiny carrying capacity and massive wings.

"In that sense, it is not the easiest way to fly," he said. "But it is the most fabulous way to fly, because the more you fly, the more energy you have on board."

He added: "We want to inspire as many people as possible to have that same spirit: to dare, to innovate, to invent."

The plane's four large batteries, attached to the bottom of the wings along with the plane's tiny motors, account for a quarter of its overall heft.

The aircraft's lightweight carbon fiber design and wingspan allow it to conserve energy, but also make the plane vulnerable to being tipped over.

A ground team of weather specialists, air traffic controllers and engineers track the plane's speed and battery levels and help the pilot steer clear of turbulence. Solar Impulse cannot fly in strong wind, fog, rain or clouds. Its machinery is not even designed to withstand moisture.

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